In a large bowl, mix together chicken, pork, beef, panko, sake, soy sauce, green onion, shiitakes, ginger juice, mirin, shiso, salt, sugar, and ichimi, being careful not to overwork.
Form into 72 (1-ounce) meatballs. Refrigerate.
In large pot, combine kombu and water. Bring to just below a simmer; remove kombu. Bring to a simmer; add half of the meatballs. Simmer 8 to 10 minutes, or until cooked through.
Remove with slotted spoon. Poach remaining meatballs the same way. Cool.
When ready to assemble, thread 3 meatballs onto each skewer. Refrigerate until needed.
In a saucepan, bring soy sauce, mirin, sake, and sugar to a boil; reduce heat and simmer until thickened. Remove from heat; stir in yuzu juice.
Over charcoal, grill several skewers, turning frequently, about 5 minutes or until lightly browned. Brush with 1 tablespoon yuzu sauce for each skewer. Serve with 1½ tablespoons sauce per skewer on the side.